Voyager 1 is flying above the plane of the planets and is now nearly 15 billion km (nine billion miles) from the Sun - nearly 100 times the Earth-Sun distance. Voyager 2 is south of the plane and a little under 12 billion km out (seven billion miles).

Both are investigating the extent of the heliosphere, the huge "bubble" within which the Sun dominates.

It contains electrically charged particles that have been blown off the Sun at high speed and which are now pressing up against matter from other stars.

This Hubble picture shows how a star imprints itself on local space

Voyager 1 crossed a turbulent zone known as the termination shock in December 2004.

At the termination shock, the solar wind is slowed abruptly from its average speed of 300-700 km/s (190-430 miles per second) and becomes denser and hotter.

Scientists were able to report at the meeting that Voyager 2, although a good distance behind Voyager 1, is already seeing signs that it is coming up on the shock. It may even pass through it in the next year.

That Voyager 2 should get up to the shock so soon indicated the heliosphere was a different shape south of the planetary plane compared with the north, the meeting was told.

Long goodbye

Scientists believe that the observed discrepancies may be attributed to an interstellar magnetic field pressing inward on the southern hemisphere.

"The Voyager 1 terminal shock was about 90 astronomical units - 8.4 billion miles - from the Sun. Down south, the shock is somewhere between 80 and 84 astronomical units - 0.6 to one billion miles closer to the Sun than in the north," said Stone.

"Voyager 2 is now very close to the shock and should have crossed it by this time next year, if our estimates are correct."

Going beyond the termination shock takes the spacecraft ever closer to the "official" edge of the Solar System - the heliopause. Past this point, the Voyager probes would be in interstellar space and, by that stage, probably some 40 years on from their launches.

Glossary of terms that describe features of the heliosphere:Solar wind: Stream of charged particles blown off the Sun and travelling at "supersonic" speedsTermination shock: Area where particles from the Sun begin to slow and clash with matter from deep spaceHeliosheath: A vast, turbulent expanse where the solar wind piles up as it presses outward against interstellar matterHeliopause: The boundary between the solar wind and the interstellar wind, where the pressure of both are in balanceInterstellar wind: The gas and dust between the starsBow shock: The shock wave caused by the edge our Solar System travelling through deep space